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Rapid eye movement, or REM sleep, is the final phase of the four stage cycle that occurs during sleep. Unlike non-REM sleep, the fourth phase is characterized by an increase in brain activity and autonomic nervous system functions, which are closer to what is seen during the awakened state. Similar to non-REM sleep stages, this stage of sleep is primarily controlled by the brainstem and hypothalamus, with added contributions from the hippocampus and amygdala. Additionally, REM sleep is associated with an increase in occurrence of vivid dreams. While non-REM sleep has been associated with rest and recovery, the purpose and benefits of REM sleep are still unknown. However, many theories hypothesize that REM sleep is useful for learning and memory formation.

Key Takeaways: What Is REM Sleep?

• REM sleep is an active stage of sleep characterized by increased brain wave activity, return to awake state autonomic functions, and dreams with associated paralysis.
• The brainstem, particularly the pons and midbrain, and the hypothalamus are key areas of the brain that control REM sleep with hormone-secreting “REM-on” and “REM-off” cells.
• The most vivid, elaborate, and emotional dreams occur during REM sleep.
• The benefits of REM sleep are uncertain, but may be related to learning and storage of memory.

REM Definition

REM sleep is often described as a “paradoxical” sleep state due to its increased activity after non-REM sleep. The three prior stages of sleep, known as non-REM or N1, N2, and N3, occur initially during the sleep cycle to progressively slow bodily functions and brain activity. However, after the occurrence of N3 sleep (the deepest stage of sleep), the brain signals for the onset of a more aroused state. As the name implies, the eyes move rapidly sideways during REM sleep. Autonomic functions such as heart rate, respiratory rate, and blood pressure begin to increase closer to their values while awake. However, because this period is often associated with dreams, major limb muscle activities are temporarily paralyzed. Twitching can still be observed in smaller muscle groups.

During REM sleep, brain wave activity measured on an electroencephalogram (EEG) also increases, as compared to the slower wave activity seen during non-REM sleep. N1 sleep shows slowing of the normal alpha wave pattern noted during the awake state. N2 sleep introduces K waves, or long, high-voltage waves lasting up to 1 second, and sleep spindles, or periods of low voltage and high-frequency spikes. N3 sleep is characterized by delta waves, or high voltage, slow, and irregular activity. However, EEGs obtained during REM sleep show sleep patterns with low voltage and fast waves, some alpha waves, and muscle twitch spikes associated with transmitted rapid eye movement. These readings are also more variable than those observed during non-REM sleep, with random spiking patterns at times fluctuating more than activity seen while awake.

REM and Your Brain

During REM sleep, brain wave activity measured on an electroencephalogram (EEG) also increases, as compared to the slower wave activity seen during non-REM sleep. N1 sleep shows slowing of the normal alpha wave pattern noted during the awake state. N2 sleep introduces K waves, or long, high-voltage waves lasting up to 1 second, and sleep spindles, or periods of low voltage and high-frequency spikes. N3 sleep is characterized by delta waves, or high voltage, slow, and irregular activity. However, EEGs obtained during REM sleep show sleep patterns with low voltage and fast waves, some alpha waves, and muscle twitch spikes associated with transmitted rapid eye movement. These readings are also more variable than those observed during non-REM sleep, with random spiking patterns at times fluctuating more than activity seen while awake.

The major portions of the brain activated during REM sleep are the brainstem and the hypothalamus. The pons and midbrain, in particular, and the hypothalamus contain specialized cells known as “REM-on” and “REM-off” cells. To induce the transition to REM sleep, REM-on cells secrete hormones such as GABA, acetylcholine, and glutamate to instruct the onset of rapid eye movements, muscle activity suppression, and autonomic changes. REM-off cells, as their name implies, induce the offset of REM sleep by secretion of stimulatory hormones such as norepinephrine, epinephrine, and histamine.

The hypothalamus also contains stimulatory cells known as orexin neurons, which secrete the hormone orexin. This hormone is necessary for maintaining wakefulness and arousal from sleep and is often decreased or absent in people with sleep disorders. The hippocampus and amygdala are also involved in REM sleep, specifically during periods of dreams. These areas of the brain are most notable for their functions in memory and emotional regulation. An EEG will show increased hippocampal and amygdala activity with the presence of high voltage, regular waves known as theta waves. 

Dreams and REM Sleep

Although dreams can occur in other stages of sleep, the most vivid dreams occur during REM sleep. These dreams are often elaborate and emotional experiences of imagined life, most often associated with sadness, anger, apprehension, or fear. A person can also more readily recall a dream when awakened from REM sleep rather than from non-REM sleep. The purpose of dream content is not currently understood. Historically, neurologist and father of psychoanalysis Sigmund Freud suggested that dreams were a representation of unconscious thought, and therefore each dream had deeply significant meaning. His dream interpretation, however, is not a universally accepted theory. An opposing hypothesis proposes that dream content is a result of random brain activity that occurs during REM sleep, rather than a meaningful interpretive experience.

Benefits of REM Sleep

Sleep in general is necessary for health and well-being, as mild sleep deprivation increases risk for chronic health conditions and severe sleep deprivation can lead to hallucinations or even death. While non-REM sleep is required in order to survive, the benefits of REM sleep remain inconclusive. Studies in which participants were deprived of REM sleep by waking have shown no obvious adverse effects. Some drugs, including MAO antidepressants, lead to drastically decreased REM sleep without issue for patients even after years of treatment.

Due to the lack of conclusive evidence, many hypotheses exist concerning the benefits of REM sleep. One hypothesized benefit relates to the association of REM sleep and dreams. This theory suggests that certain negative behaviors, which should be “unlearned,” are rehearsed through dreams. Actions, events, and sequences related to fearful situations are often the subject of dreams and are therefore appropriately erased from the neural network. REM sleep is also proposed to help transfer memories from the hippocampus to the cerebral cortex. In fact, the cyclical occurrence of non-REM and REM sleep is often thought to enhance the body’s physical and mental rest as well as aid in memory formation.

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REM sleep is an active stage of sleep characterized by increased brain wave activity. Jamie Grill / Getty Images

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